Year	Percent
1981	46.06
1982	47.1
1983	47.58
1984	48.42
1985	49.28
1986	49.99
1987	51.59
1988	52.42
1989	53.17
1990	54.47
1992	51.36
1993	51.89
1995	55.91
1996	56.86
1997	57.06
1998	57.73
1999	58.01
2001	58.78
2002	58.4
2003	58.77
2004	58.17
2005	58.36
2006	59.39
2007	59.77
2008	59.87
2009	60.82
2011	62.7
2012	65.47
2013	65.86
2014	66.73
2015	67.59
2016	68.93
2017	70.01